We investigate lateral migration of particles dispersed in Stokes fluid. Arbitrary shaped and non-uniform charge distributed particles are discussed in this paper. Translation-rotation coupling of the particle sometimes produces the complex motions. The motions of the particles are not always parallel to external fields, such as gravity, electric field and shear flow. The migration direction and the trajectories of the particles are dependent on their shapes etc. Especially chiral particles, which cannot be superimposed to their mirror images, migrate to opposite direction of their mirror images isomer. The topics here can be applied to the separation method of the particles.

In recent years, self-assembly of complex fluids has received considerable attention in the fields of nanoscience and nanotechnology. Self-assembly is involved in various functional materials that are widely used in industry and there is a strong relationship between self-assembled structures and specific functions. Hence, the understanding and controlling of the self-assembly controlling of complex fluids is important and a useful process in materials chemistry and engineering. In this paper, the recent developments on studies of the structure formation and flow properties of amphiphilic molecules and Janus colloids in solution, using coarse-grained simulation that takes into account hydrodynamic interactions are reviewed. We first briefly introduce the dissipative particle dynamics method and discuss in detail the self-assembly behavior and viscosity properties of surfactant aqueous solution in nanotubes. Following this, we describe another simulation method in which solvent particles are modeled explicitly as point particles, which is the so-called “the multi-particle collision dynamics (MPCD)". The relation between the structural and rheological properties of Janus colloids in solution, using molecular dynamics simulations coupled to MPCD is reported. In general, the shear viscosity of the dispersion correlates strongly with the size of the aggregates not aggregates shape. These results allow us to determine the dynamic properties solely based on structural information.

Capsule refers to a particle consisting of a liquid droplet enclosed by a thin-membrane. Analyzing the behavior of capsules in flow is needed to predict the transport and breakup of the capsules, and to understand the rheological property of the capsule suspension. We have developed a graphics processing unit (GPU) accelerated boundary element method (BEM) for the fluid-structure interaction analysis of capsule suspensions at the Stokes flow regime, where BEM for fluid mechanics is coupled with the finite element method (FEM) for membrane mechanics. Here, we provide an overview of the GPU-accelerated BEM and its application to the rheological analysis of capsules suspensions.

The numerical analysis of flow-induced structures of complex fluids is one of effective approaches for elucidating the mechanism of their complicated flow behavior. In this article, three examples of the numerical simulation of the flow-induced structure using the Brownian dynamics (BD) method and the multi-particle collision dynamics (MPCD) method are presented: (a) BD simulation of shear flows of polymer/clay suspensions, (b) MPCD simulation of shear flows of linear polymer solutions, and (c) MPCD simulation of bio-convection of phototactic microalgae in water. In these simulations, complex fluids are modeled by dispersion systems of basic elements of the fluid inner structure such as particles and model polymers, and their behavior is analyzed for the investigation of the flow-induced structure.

A Steam Injector (SI) has capability to be applied to heat pump systems and to have downsized them. A SI works as passive jet pump and heat exchanger without external power sources. It has high heat-transfer performance due to direct contact condensation between steam flow and subcooled water jet. The objective of the present study is to clarify thermal hydraulics characteristics and operating criteria for an Ultra-Micro Steam Injector. To this end, SI with throat diameter of 2.0 mm was designed and investigated experimentally. Visualization of internal flow behavior and measurement of pressure and temperature were conducted. As a result, we confirmed that the formation of the water jet, the stop of drainage from the drain port and the increase of discharge pressure. The internal flow patterns were classified into four types and considering the energy balance of water and steam. One of the operating criteria was clarified that water could completely condense the steam. The discharge pressure was increased up to 2 times at maximum. There is possibility the operating criteria can be predicted from the position where the flow behavior changes.

Cleaning technology is widely used to remove oil or resist from metal and semiconductor and soon. However, it causes high cost and severe environmental impacts because of the sewage disposal. Owing to strong oxidizing ability and self-decomposing power, ozone micro-bubbles generated by a Venturi tube was proposed. The purpose of this study is to research the capability of ozone micro-bubbles and improve the cleaning mechanism. In this study, we focus on the microbubble generator with a Venturi tube. We visualized the Venturi Tube by a high-speed camera to measure the bubble diameter. We also measure the shear stress near the test piece surface by using PIV. And by titration with Potassium iodide, ozonated water and ozone gas concentration are measured. Ozone gas is well dissolved after passing through the Venturi tube. Also, brass test pieces applied with cutting oil and latex resin are washed by using bubbly flow with ozone micro-bubbles. It has been confirmed that ozone micro-bubbles are able to clean the test pieces efficiently.

Compulsive sedimentation of soil particles using coagulant is one of the countermeasures for long-term turbid water in a dam reservoir. We used allophane as a coagulant, since it is considering that allophane obtained from nature has low environmental impact when it is added into the dam reservoir. The objective of the present study is to propose an atomization and transportation technique for allophane with a Venturi tube and to elucidate their mechanisms. We successfully visualized atomization behavior of allophane in detail and considered the atomization mechanisms. In addition, we confirmed that the turbidity of the water is improved by adding atomized allophane. In transportation experiment of allophane, it is found that allophane transportation is promoted in the condition of microbubble generation. Furthermore, visualization of allophane adhering to microbubble and following water flow is achieved, and it is suggested that allophane is transported by microbubble. Finally, we measured zeta potential of particles that is important factor in allophane transportation, and tried to elucidate the transportation mechanisms with evaluating the relationship with Sauter mean diameter of allophane.

Nowadays high performance cooling equipment are required for high heat dissipation electric devices. In this research, the experiments were carried out with high porosity sintered fiber on the surface under the forced convection subcooled boiling heat transfer. The experimental parameter of the porous is the filling height relative to the channel height, and the porosity of porous material was 86 %. According to the experiments, all of porous surfaces, almost 2 times higher performance on boiling heat transfer compared with the bare surface under the same wall superheat ⊿T_sat (K) conditions. However, the high height porous condition, the CHF tend to low compared with lower height porous condition and bare surface. In addition, the relations between the bubble behavior and the heat transfer were analyzed from experimental results and the videos acquired by high speed camera.

Minimum breakup length of gas-liquid two-phase jet was explored experimentally since it affects the droplet carryover characteristics of gas-liquid separators used in various industrial plants. In the experiments, air and water were used as the test liquids, and the two-phase jet was discharged vertically downward from a circular nozzle. An annular liquid film was formed at the nozzle exit. The deformation of the liquid film became significant with an increase in the gas flow rate, suggesting that the deformation was caused mainly by the shearing force exerted by the high-speed gas core flow. The breakup of liquid film occurred following the deformation. The minimum breakup length was fairly proportional to the liquid film thickness evaluated at the nozzle exit. The average proportionality factor was 59 in the present experiments.

In our previous study (Goda et al.), we measured the void fraction α, pressure gradient dP/dz, and countercurrent flow limitation (i.e. relationship between superficial velocities, J_G and J_L) in vertical pipes (diameter D = 20 and 40 mm) under flooding conditions at the square bottom end and working fluids of air and water to obtain the wall friction factor f_w and the interfacial friction factor f_i based on the annular flow model. Measurements of α in vertical pipes under flooding conditions are few. In this study, therefore, we evaluated f_i from the measured dP/dz under flooding at the square bottom end reported by Bharathan et al. with D = 6.4 to 152 mm and air-water and by Ilyukhin et al. with D = 20 mm and working fluids of steam and water at pressures of P = 0.6 to 4.1 MPa. As a result, we found that the f_i values obtained from the measured dP/dz and the f_w correlation by Goda et al. were well correlated in terms of the gas Kutateladze parameter K_G* and the dimensionless diameter D*. The effect of fluid properties was expressed by a revision term with the viscosity ratio of gas and liquid phases (μ_G/μ_L).

The length of liquid slug and gas bubble, gas bubble velocity and pressure drop of gas-liquid two-phase flow in a rectangular channel were measured using a high speed camera and a different pressure transducer. Water and Glycerol-solution as Newtonian liquids and Sodium carboxymethyl cellulose, Xanthan gum and Polyacrylamide-water solution as non-Newtonian liquids were used as the liquid phase, and N₂ gas was used as the gas phase. The equivalent hydraulic diameter of the rectangular channel was 0.51 mm. The measured pressure drop data were compared with unit cell model, which applied to gas bubble pressure drop prediction proposed by Kurimoto et al. (2017). The following conclusions were obtained: (1) Different flow pattern was observed only in Polyacrylamide-water solution. (2) Frictional pressure drop on two-phase flow can associate with an effective viscosity of liquid. (3) Effects of elasticity of liquid on non-Newtonian two-phase two-phase flow should be accounted to predict accurately pressure drop.

Thermohydraulic behavior in spent fuel pool is quite important in evaluating safety of a nuclear reactor under accidental conditions. Particularly, accurate prediction of void fraction in spent fuel pool is indispensable for evaluating cooling characteristics of spent fuel. In view of these, experimental and analytical studies were carried out for void fraction in spent fuel pool. The experiment was performed to measure the heat-up and void fraction transient during the postulated SFP accident. In this experiment, a simulated 7x7 BWR rod bundle that consists of 49 heater rods, 7 spacer grids and upper tie-plate was used. The measured data was compared with the some drift-flux correlations under the low pressure and the low flow rate condition related to SFP accident.

Filling holes with liquid, or discharging gas from holes, is a fundamental process in both cleaning and painting. Discharging gas from small holes with closed end and high aspect ratio is extremely difficult due to surface tension. In this study, we developed a new gas discharge process from a hole by using acoustic wave irradiation. We irradiated two types of acoustic waves: waves with constant frequencies and those with variable frequencies in time i.e. sweep wave for test samples in a water pool. In addition, we observed gas discharge using high-speed video camera. As a result, we succeeded in completely discharging gas by using sweep frequency of the acoustic waves. From the observation results, we confirmed that gas discharge consists of three stages. The gas was discharged from the hole mainly at the first and third stage. In these stages, the natural frequency of the gas column in the hole was essential. In the second stage, the gas column in the hole breaks into multiple gas columns; then, the gas was hardly discharged even with the acoustic wave irradiation.

In the design study on sodium-cooled fast reactors, .it is important to investigate the gas entrainment (GE) phenomena in detail. In fact, a lot of analytical, experimental and numerical studies have been conducted to clarify the onset condition of GE and some GE onset models have been proposed. However, few studies on the modeling of entrained gas flow rate has been conducted due to the difficulty on modeling the gas bubble entrainment at a free surface, which is accompanied by complicated free surface deformation. In this paper, the authors propose a mechanistic model to predict the entrained gas flow rate by a free surface vortex. The model contains the theoretical equation of transient gas core elongation and the empirical equation of critical gas core length for gas bubble detachment. The mechanistic model is applied to predict the entrained gas flow rate in a simple GE experiment. As a result, the predicted results show qualitatively good agreement with the experimental results of the entrained gas flow rate. Therefore, it is confirmed that the proposed mechanistic model can predict the entrained gas flow rate by a free surface vortex.

Internal gas-liquid two-phase flow is commonly observed in various engineering disciplines. One of its unique characteristics is the presence of flow regime, and its identification is crucial for improving the accuracy of thermal-hydraulic analysis codes as well as securing the integrity of piping systems. Furthermore, transition of two-phase flow regime takes place gradually, and is highly unsteady phenomenon, objective flow regime identification near the transition region still remains challenging to this day. In the present study, we developed the novel flow regime identification method based on the state-of-the-art AI technique which is fully automated and capable of detecting bubble characteristics from high-speed images at high accuracy. The present tool was developed based on a machine learning algorithm, which can quickly detect flow characteristics and capture individual bubble positions from given images. Furthermore, it is possible to calculate major two-phase flow parameters such as void fraction and bubble rise velocity using the detection results. By utilizing the current tool, instant objective two-phase flow feature extraction is now possible, and our results showed promising performance by coupling the state-of-the-art AI technique with conventional thermal-hydraulics.

We compared the sodium-water reaction in vapor sodium phase at 1273 K with that in the sodium/metal multiphase domain by applying the accelerated quantum chemical molecular dynamics method. Considering the gas/liquid mixture condition on the sodium-water interface as well as the solid reaction products such as sodium oxides, the sodium-water reaction itself is regarded as multiphase reaction. In case of the water adsorption over the vapor sodium phase, sodium atoms lost electrons, and desorbed as hydrogen molecule. In case of the sodium vapor over the transition metal titanium surface, a pair of hydrogen atom was produced.

Weakly nonlinear propagation of plane pressure waves in a flowing water uniformlycontaining many spherical microbubbles is theoretically investigated. At the initial state, the gas and liquid phases have different flow velocity distributions as a small nonuniform effect in bubbly flows. The basic equations based on a two-fluid model are utilized to describe velocity distributions of gas and liquid phases. By using the method of multiple scales and the determination of size of three nondimensional ratios, we can systematically derive two types of nonlinear wave equations describing long-range propagation of waves. i.e., the Korteweg-de Vries-Burgers (KdVB) equation and the nonlinear Schrödinger (NLS) equation with variable coefficients. As a result, initial velocity distributions affect an advection effect of waves induced by a relative velocity between gas and liquid phases and a moving bubble.

This study theoretically clarifies an effect of the liquid viscosity and the thermal conductivity on weakly nonlinear propagation of pressure waves in a liquid containing many spherical microbubbles. As in our preceding paper (Kamei et al., J. JSCE, Ser. A2, 75 (2019), 499) focusing on a long wave, by the use of the method of multiple scales, a nonlinear Schrödinger equation describing the long-range propagation of an envelope wave of short carrier wave is derived from the basic equations incorporating the liquid viscosity and the thermal conductivity. As a result, as in our preceding long wave case, the liquid viscosity and the thermal conductivity affect the dissipation effect, and a nonlinear effect in the adiabatic process decreases in comparison with the previous study (Kanagawa et al., J. Fluid Sci. Technol., 6 (2011), 838). On the other hand, unlike in our preceding long wave case, a dispersion effect in the adiabatic process decreases in comparison with the previous study.

Oscillation of gas bubbles in a bubbly liquid induces dissipation and dispersion effectsof waves into a nonlinear evolution of pressure waves. Long-range propagation of pressure waves with a moderately small amplitude is described by the KdV-Burgers (KdVB) equation. This paper numerically solves the KdVB equation via a spectral method to predict the nonlinear evolution of waves in bubbly liquids. Focusing on the waveform, and the nonlinear, dissipation and dispersion terms, the following results are obtained: (i) An initially sinusoidal waveform satisfying a periodic boundary condition is firstly distorted due to the nonlinear effect; (ii) Wave distortion is suppressed by increasing the dissipation and dispersion effects; (iii) A break-up due to the dispersion effect appears; (iv) A balance between the nonlinear and dispersion effects is accomplished and then a pulse wave satisfying a feature of soliton is formed. As a result, the initial bubble radius and the initial void fraction strongly contribute the dissipation and dispersion effects, respectively.

Two types of weakly nonlinear propagations of plane progressive pressure waves in an initially quiescent compressible liquid uniformly containing many spherical gas bubbles are theoretically investigated. The treatment of two types of propagations corresponds to an extension of our previous result (Yoshimoto & Kanagawa, Jpn. J. Multiphase Flow, 33 (2019), 77) to a generic form. The main assumptions are as follows: (i) The incident wave frequency is much larger than an eigenfrequency of single bubble oscillations; (ii) The compressibility of the liquid phase, which has long been neglected and induces the high speed propagation mode, is considered; (iii) The wave propagates with a large phase velocity exceeding the speed of sound in pure water. From the method of multiple scales with two types of appropriate choices of three nondimensional parameters, we can systematically derive two types of nonlinear Schrödinger (NLS) equations with some correction terms in a unified way. These two equations can describe high-speed propagation of pressure waves in compressible bubbly liquids.

An approach to clarify the effect mechanism of microbubbles in various fields is shown. Fluorescence and absorption measurements were carried out on solutions of bovine serum albumin and hemoglobin. The fluorescence intensities of the proteins in a semipermeable membrane were observed in the absence and presence of air-, O₂-, or N₂-microbubbles. The microbubbles were not diffused through the semipermeable membrane from an outside to an inside solution. The air-microbubble system showed a significant decrease in the fluorescence intensity with respect to the bubbling time. This study raises the possibility that NO is produced from the reaction of nitrogen and oxygen gas in the air during crushing of microbubbles.

In recent years, significant amounts of polymerized waste, which cannot be processed using chemicals, has become problematic. Treatment of such materials with ozone and hydrogen peroxide has been reported; however, the safety of the former as well as the stability of hydrogen peroxide are not optimal. Thus, various challenges exist to improve these approaches. It is noteworthy that these methods primarily rely on the oxidation-reduction reactions involving the hydroxyl radical (OH･). Furthermore, UFB (Ultrafine Bubble) water generates negative ions, and thus, simultaneous interactions with the radical species occur. Importantly, the oxidation-reduction effect of the radical species and negative ions is largely reflected in the nano number density of UFB water. It is crucial to establish a technique, which would result in mass-production of high concentration UFB water. In the current study, we attempted to generate a large volume of stable UFB water using pressure dissolution and a stirring mixer. In 20 min, approximately 867 million UFBs /mL of air UFB water was produced. Moreover, air UFB water of approximately 512 million UFBs /mL resulted in OH radical waveform. Air UFB water of approximately 868 million UFBs /mL was in OH radical waveform, which was stronger than air UFB water of approximately 512 million UFBs /mL. Thus, the intensity of the OH radical appeared to increase with the increase in the nano number density of UFB water.

The test method for evaluating the promotion effect of Ultrafine Bubble (UFB) on barley seed germination was published as an ISO standard in July 2019. There should be a minimum number concentration of UFB that can provoke early seed germination, but it is still unclear. Hence, we conducted a germination examination by applying UFB water containing different number concentrations. The results showed that the promotion of germination is observed statistically with UFB water whose number concentration is in the range of 10 to the 8^th power/ml. The difference in the number concentration of UFB was also supported by ESR analysis. After the application of ultrasonic sound of 43 kHz for 30 s, UFB water containing UFB of 6.03 x 10⁸ particles/mL showed the larger signal intensity of reactive oxygen species which was composed more of hydroxyl radicals than that of 1.26 x 10⁸ particles/mL. This fact indicated the difference in the number concentration of UFB without any effect of foreign matter included in UFB water.

To gain insight into the mechanism of texture formation of bubbles in a glass of Guinness beer, a numerical simulation of an unsteady flow in an inclined vessel is performed using Eulerian-Lagrangian approach with discrete element method. The texture formation can be reproduced by quasi-two-dimensional simulation. The velocity fluctuation is enhanced with increasing the initial particle volume fraction and with decreasing container inclination angle. Arranged using the Froude number Fr and the inverse resolution of concentration interface H, the numerical results of the velocity fluctuation magnitude strengthen the experimentally inferred argument that the texture formation is caused by a roll wave instability in the pure fluid layer, which postulates fluid-like behavior of the dispersed phase.

Two-phase flow pattern of gas-liquid motion driven by a rotating cylinder is numerically studied. Two-dimensional direct numerical simulations are performed using a fixed mesh approach based on Volume Of Fluid (VOF) and Boundary Data Immersion (BDI) methods. With varying rotation speed Ω, material properties, and radius ratio, the distribution of the computed VOF function is classified into four patterns with respect to the liquid film thickness and liquid fraction on the rotating cylinder wall. The relevance of the flow pattern to the torque is discussed. The liquid film thickness at low Ω is found to be scaled using a ratio of viscous force to gravitational force.

The Marangoni effect on the repulsive forces on approaching interfaces, which is likely to suppress bubble coalescence due to the surfactant addition, is numerically studied. We consider a bubble-wall system and a bubble-bubble one. We perform direct numerical simulation using a boundary-fitted grid conforming to the interfacial deformation. Spatiotemporal development of the Marangoni stress and liquid film drainage are investigated for various Reynolds and Marangoni numbers. The change in the gas-liquid interface shape is categorized into three types: continuation of pimple, continuation of dimple, and repetition of pimple and dimple. When a dimple forms, the Marangoni stress induced by interface deformation suppresses the pressure drop along the film and therefore the displacement at dimple head, accounting for the mechanism of suppressing bubble coalescence.

Contact line can be easily stuck (pinned) on the defects or boundary of the different wettability regions. The contact line remains to be trapped until the contact angle increases beyond the advancing angle or decreases below the receding angle. Once such condition is satisfied, the contact line is released from the trapping site and rapidly proceeds or recedes with the contact angle changing toward the equilibrium contact angle. In this study theoretical description on the contact line motion and the contact angle after the release from the stuck condition at the groove edge is developed. The normal stress balance and the corresponding flow field adjacent to the surface are expressed under Stokes approximation. Since the interface deformation was limited within a certain distance from the solid surface, referred as ‘boundary layer’, the equation on the normal stress balance equation was solved with the boundary condition at the contact line and the outer boundary of the boundary layer. The results numerically obtained were compared with the experimental data, showing fairly good agreement between them.

This study aims to reveal the development process of artificial void waves generated in a fully developed turbulent channel flow. Void waves generated with periodically fluctuated air flow rate were optically visualized by a high-speed video camera at three locations along the channel. The measured spatio-temporal fluctuations of void waves maintained the initial frequency given at the injection during the downstream propagation. The diffusion coefficients of void distributions were calculated to evaluate the influence of diffusion in a two-phase turbulent flow. The coefficients denoted that diffusion effect was larger than that of turbulent eddy viscosity on void wave deformation.

We washed a soiled stainless-steel (SUS304) surface with attached starch paste as the soiled model because of investigating washing effect of microbubble mixtures generated by a swirling method (S-MB), a pressurized method (P-MB), and a pressurized method with a pressurized tank (NP-MB). Compared with the resultant washing rate of tap water alone, relatively high washing effect of microbubble mixtures was obtained. Furthermore, the washing effect of P-MB approximately agreed with those of S-MB. For understanding the experimental results, particle diameter change against elapsed time and static contact angle of interface between solid surface and droplet of test fluids was discussed. Particle diameters of all microbubble mixtures were changed as elapsed time. Elapsed time until collapse of P-MB higher than that of S-MB. Moreover, NP-MB exhibited highest elapsed time until collapse. Moreover, static contact angle of microbubble mixtures was lower than that of tap water alone. Correlation between washing effect and contact angle was obtained. The obtained knowledges were beneficial for many applications using fine bubble mixtures.